Recording apparatus

ABSTRACT

A recording apparatus, which records information in an optical recording medium, includes: a mode-lock laser unit including a saturable absorber section that applies a bias voltage, a gain section that feeds a gain current, a semiconductor laser that emits laser light used to record the information on the optical recording medium, and an external resonator; an optical modulation unit performing amplification modulation on the laser light emitted from the mode-lock laser unit; a reference signal generation unit generating a master clock signal and supplying a signal synchronized with the master clock signal to the gain section of the semiconductor laser; a recording signal generation unit generating a recoding signal based on the master clock signal; and a driving circuit generating a driving pulse used to drive the optical modulation unit based on the recording signal.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Japanese Priority PatentApplication JP 2010-184594 filed in the Japan Patent Office on Aug. 20,2010, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present application relates to a recording apparatus that uses anMOPA (Master Oscillator Power Amplifier) in which a mode-lockoscillation type laser and an optical amplifier are combined as arecording optical source.

High-peak power laser light, particularly, ultra-short pulse light, isvery effective for realizing the procedure of nonlinear multi-photonabsorption.

This procedure of the nonlinear multi-photon absorption is expected tobe applied to three-dimensional optical recording, ultra-fineprocessing, non-destructive bio-imaging, or the like.

For example, there has been reported a method of realizing multi-layerrecording by emitting high-power laser light to a transparent bulkmaterial having a nonlinear effect (see ISOM2009 Digest Th-1-01, 2009 bySeiji Kobayashi, Kimihiro Saito, Takashi Iwamura, Hisayuki Yamatsu,Toshihiro Horigome, Mitsuaki Oyamada, Kunihiko Hayashi, Daisuke Ueda,Norihiro Tanabe and Hirotaka Miyamoto).

According to this method, recording on a large-capacity recording mediumcan be realized more cheaply than a lamination type disc according tothe related art.

A mode-lock type titanium-sapphire laser has been used as an opticalsource that outputs high-power laser light. Even in the example ofISOM2009 Digest Th-1-01, 2009 by Seiji Kobayashi, Kimihiro Saito,Takashi Iwamura, Hisayuki Yamatsu, Toshihiro Horigome, Mitsuaki Oyamada,Kunihiko Hayashi, Daisuke Ueda, Norihiro Tanabe and Hirotaka Miyamoto,emission light of a titanium-sapphire laser with 810 nm is convertedinto light with a 405 nm wavelength by an SHG (Second HarmonicGenerator) and is used in a short-wavelength recording optical sourceadvantageous for high-density recording.

Such a large expensive solid-state laser is limited to applications toexperiments in laboratories (for example, see Spectra-Physics Inc.Online (searched in Aug. 6, 2010), Internet i_Series_Data_Sheet.pdf>).

For this reason, many researchers have tried to develop a small-sizedand stable pulse optical source as a semiconductor base to put topractical use.

In the next-generation optical recording, as in the above-mentionedmethod, a blue-purple laser optical source advantageous for thehigh-density recording of all semiconductors is strongly preferred.

For example, in a gain switching type laser, it has been reported thatwhen strongly excited driving is repeatedly performed at 1 MHz, the peakpower of 55 W has been realized (see Appl. Phys. Lett. 96, 051102, 2010by M. Kuramoto, T. Oki, T. Sugahara, S. Kono, M. Ikeda, and H.Yokoyama).

However, a higher repetition frequency is necessary even in a datarecording optical source according to a request for a high datatransmission rate in the market.

In recent years, it has been reported that an optical source of 100 Whas been realized at a repetition frequency of 1 GHz by a blue lasereffective for high-density recording (for example, see APPLIED PHYSICSLETTERS 97, 021101, 2010 by Rintaro Koda, Tomoyuki Oki, Takao Miyajima,Hideki Watanabe, Masaru Kuramoto, Masao Ikeda, and Hiroyuki Yokoyama).

This optical source has a configuration called an MOPA (MasterOscillator Power Amplifier) in which a semiconductor mode-lock laser anda semiconductor optical amplifier are combined.

A recording reproduction apparatus has to record data used in recordingat an arbitrarily position based on a wobble signal read from an opticalrecording medium.

At this time, it is necessary to modulate the recording data insynchronization with an oscillation pulse.

SUMMARY

When the MOPA using the mode-lock oscillation type laser is applied to arecording reproduction apparatus, it is possible to modulate therecording data by outside driving of an optical amplifier which is apower amplifier.

However, since the optical oscillation frequency of the mode-lockoscillation type laser is generally determined as a unique frequency bythe resonant length of a spatial resonator, the optical oscillationfrequency is not synchronized with the outside driving of the opticalamplifier. Even when the optical oscillation frequency matches with thefrequency of a master clock of a recording system used for the outsidedriving by adjusting the resonant length, phase synchronization may notbe achieved.

For this reason, the following problem may arise.

For example, it is supposed that the optical amplifier is driven withthe master clock and a pulse for the recording data corresponding to a 5T mark is amplified. When the pulse light from the mode-lock oscillationtype laser incident on the optical amplifier is synchronized with thedriving of the optical amplifier in the optimum phase, a pulse signal isaccurately generated.

However, when the pulse light incident on the optical amplifier is notsynchronized with the master clock or the phase is not optimum, thepulse via the optical amplifier may become a signal corresponding to a 4T mark from the signal corresponding to the original 5 T mark (see FIG.5). For this reason, the signal may not be accurately generated.

It is desirable to provide a recording apparatus capable of easilyobtaining synchronization of modulation of an optical pulse of laserlight and the laser light with a simple configuration.

According to an embodiment, a recording apparatus is a recordingapparatus that records information on an optical recording medium.

The recording apparatus includes a mode-lock laser unit including asaturable absorber section that applies a bias voltage, a gain sectionthat feeds a gain current, a semiconductor laser that emits laser lightused to record the information on the optical recording medium, and anexternal resonator.

The recording apparatus further includes an optical modulation unitperforming amplification modulation on the laser light emitted from themode-lock laser unit and a reference signal generation unit generating amaster clock signal and supplying a signal synchronized with the masterclock signal to the gain section of the semiconductor laser.

The recording apparatus further includes a recording signal generationunit generating a recoding signal based on the master clock signal and adriving circuit generating a driving pulse used to drive the opticalmodulation unit based on the recording signal.

In the recording apparatus according to the embodiment, the signalsynchronized with the master clock signal is supplied from the referencesignal generation unit to the gain section of the semiconductor laser.Thus, the optical pulse of the laser light emitted from the mode-locklaser unit including the semiconductor laser and the master clock signalcan be synchronized with each other.

The recording signal generation unit generates the driving pulse basedon the master clock signal and drives the optical modulation unitmodulating the laser light based on the driving pulse. Thus, since theoptical modulation unit is driven to be turned on or off insynchronization with the master clock signal, the laser light ismodulated in synchronization with the master clock signal.

Due to the synchronization with the master clock signal, the opticalpulse of the laser light and the modulation of the laser light can besynchronized with each other.

According to the above-described embodiment, since the optical pulse ofthe laser light and the modulation of the laser light can besynchronized with each other, the optical pulse of the laser light andthe modulation of the laser light can easily be synchronized with eachother even when the laser light has a very high pulse optical frequency.

Accordingly, the recording can be accurately realized with a highdensity and at a high speed.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram illustrating the configuration of arecording apparatus according to a first embodiment;

FIGS. 2A and 2B are schematic diagrams illustrating an example of asemiconductor laser in FIG. 1;

FIG. 3 is a schematic diagram illustrating an example of a semiconductoroptical amplifier in FIG. 1;

FIGS. 4A and 4B are diagram illustrating input light and output light ofthe semiconductor optical amplifier in FIG. 1;

FIGS. 5A to 5D are diagrams illustrating a problem at a risingpoint/falling point of a driving pulse of the semiconductor opticalamplifier;

FIG. 6 is a schematic diagram illustrating the configuration of arecording apparatus according to a second embodiment;

FIGS. 7A to 7E are diagrams illustrating phase adjustment performed by aphase adjustment circuit in FIG. 6;

FIG. 8 is a diagram illustrating a case where zoning is performed in anoptical recording medium;

FIG. 9 is a diagram illustrating a relationship between a clockfrequency and a clock density of each zone in an example of Table 1 andan example of Table 2; and

FIG. 10 is a schematic diagram illustrating the configuration of arecording apparatus according to a third embodiment.

DETAILED DESCRIPTION

Embodiments of the present application will be described below in detailwith reference to the drawings.

The description thereof will be made in the following order.

1. First Embodiment

2. Second Embodiment

3. Third Embodiment

1. First Embodiment

FIG. 1 is a schematic diagram illustrating the configuration of arecording apparatus according to a first embodiment.

A recoding apparatus 200 shown in FIG. 1 includes a mode-lock laser unit210, an optical isolator unit 220, an optical amplifier unit (SOA unit)230, and a beam shaping unit 240.

The recording apparatus 200 further includes a bias Tee formed by avariable voltage source 12, a coil 13, and a capacitor 14, a referencesignal generation unit 15, a data generation unit (recording signalgeneration unit) 16, an SOA drive circuit 17, a spindle circuit 18, anda spindle motor 20.

Further, the recording apparatus 200 includes various kinds of circuitsor various kinds of optical components (none of which are shown).

The mode-lock laser unit 210 includes a semiconductor laser 1 andoptical components such as a lens 3, a band-pass filter 4, and a mirror5 passing through laser light emitted from the semiconductor laser 1.The band-pass filter 4 passes through light with a given wavelengthrange and does not pass through light with a wavelength out of thewavelength range.

An external resonator (spatial resonator) is disposed between a mirrorof the rear end surface of the semiconductor laser 1 and the mirror 5.The frequency of the laser light emitted from the mode-lock laser unit210 is determined in accordance with the path length of the externalresonator. Thus, since the frequency of the laser light can be forciblylocked to a specific frequency, the mode of the laser light can belocked.

An exemplary configuration of the semiconductor laser 1 shown in FIG. 1is schematically illustrated in FIGS. 2A and 2B. FIG. 2A is aperspective view of the semiconductor laser and FIG. 2B is a schematicview of the laser light emitted from the semiconductor laser.

The semiconductor laser 1 includes a gain section 116 and a saturableabsorber section 117, as shown in FIGS. 2A and 2B. That is, thesemiconductor laser 1 is a BS (bisectional) type semiconductor laser.

When the saturable absorber section 117 is provided, an absorption rateis lowered with an increase in the intensity of light incident on theabsorber section. Accordingly, since only a pulse with a large intensitycan be transmitted, a narrower pulse can be obtained.

Further, a gain current is fed to the gain section 116.

An n-type GaN layer 103, an n-type clad layer 104, an active layer 105,a p-type electron barrier layer 106, and a p-type clad layer 107 arelaminated on an n-type GaN substrate 102.

As shown in FIG. 2A, a ridge structure is formed in the middle of thep-type clad layer 107. A SiO2 layer 108 and a Si layer 109 are formed onthe ridge side surfaces or a portion where the ridge structure of thep-type clad layer 107 is not formed.

On the p-type clad layer 107 and the Si layer 109, p-type electrodes 113and 114 are formed by an ohmic contact.

That is, a main electrode 113 is formed on the gain section 116 and asub-electrode 114 is formed on the saturable absorber section 117. Sincethe electrodes 113 and 114 are separated by a separation section 115formed with a groove with a width of, for example, 20 μm, the electrodes113 and 114 are electrically isolated from each other. The lengths ofthe main electrode 113 and the sub-electrode 114 are, for example, 520μm and 60 μm respectively.

A lower electrode 101 is formed by an ohmic contact on the lower surfaceof the n-type GaN substrate 102.

An anti-reflection film (see FIG. 2B) 118 is coated on the cleaved planeof the front surface of the gain section 116. A high reflection film 119(see FIG. 2B) is coated on the cleaved plane of the rear surface of thesaturable absorber section 117.

In the semiconductor laser 1, as shown in FIG. 2B, a reverse biasvoltage Vsa is applied to the saturable absorber section 117 by thesub-electrode 114. At this time, by feeding a current I from the mainelectrode 113 to the gain section 116, the laser light is emitted in adirection indicated by an arrow A1.

In the recording apparatus 200 according to this embodiment, theconfiguration of the used semiconductor laser 1 is not limited to theconfiguration of the semiconductor laser 1 shown in FIGS. 2A and 2B, buta semiconductor laser with another configuration can be used.

The semiconductor material of the semiconductor laser 1 is selecteddepending on the wavelength of the laser light used for recordinginformation in the recording apparatus 200.

The optical isolator unit 220 is disposed on the rear stage of themode-lock laser unit 210 with a lens 6 interposed therebetween.

The optical isolator unit 220 includes an optical isolator 7 and amirror 8.

The optical isolator 7 has a function of preventing light reflected fromoptical components or the like on the rear stage from being incident onthe semiconductor laser 1.

The optical amplifier unit (SOA unit) 230 serves as an opticalmodulation unit that performs amplification modulation on the laserlight emitted from the semiconductor laser 1. The optical amplifier unit230 is disposed on the rear stage of the optical isolator unit 220 witha lens 9 interposed therebetween.

The optical amplifier unit (SOA unit) 230 is configured by an SOA(Semiconductor Optical Amp), that is, a semiconductor optical amplifier2.

The semiconductor optical amplifier 2 is a small-sized and cheap opticalamplifier and can be used as an optical gate or a light switch thatturns light on or off.

In this embodiment, the laser light emitted from the semiconductor laser1 is modulated when the semiconductor optical amplifier 2 is turned onor off

An exemplary configuration of the semiconductor optical amplifier 2shown in FIG. 1 is schematically illustrated in FIG. 3.

In a general semiconductor laser, light is confined in a resonator thatis configured by mirrors on the both end surfaces and laser light isoscillated by optical gain due to inter-band transition.

Meanwhile, as shown in FIG. 3, instead of the mirrors, anti-reflectionfilms 25 are disposed on the both end surfaces of the semiconductoroptical amplifier 2 to suppress the oscillation of the laser light, sothat the semiconductor optical amplifier 2 is operated as an amplifiercorresponding to one pass.

In the semiconductor optical amplifier 2, a semiconductor layerincluding an active layer 24 is configured to be laminated, as in thesemiconductor laser.

In the semiconductor optical amplifier 2, an upper electrode 22 isformed on the upper surface and a lower electrode 23 is formed on thelower surface. The upper electrode 22 is connected to a current source26 and the lower electrode 23 is a connected to a ground potential.

When a driving current is fed from the current source 26 to the upperelectrode 22 and the laser light is incident from the incident endsurface on which the anti-reflection film 25 is formed, the laser lightis amplified by induced emission during wave-guiding of the laser lightin the active layer 24.

By controlling the amount of driving current fed to the semiconductoroptical amplifier 2, the amplification amount of the laser light can becontrolled.

However, the laser light incident to the semiconductor optical amplifier2 may not be necessarily amplified. When a sufficient laser light powercan be obtained, the gain of the semiconductor optical amplifier 2 maybe set to 1.

In the semiconductor optical amplifier 2 with the above-describedconfiguration, since a carrier lifespan is short, high-speed response ismade for a variation in the current or the optical intensity.Accordingly, as for continuous pulse light input from the semiconductorlaser 1, as shown in FIG. 4A, for example, pulse light with a waveformshown in FIG. 4B can be obtained as light output from the semiconductoroptical amplifier 2.

That is, since ON and OFF can be controlled by the signal of the drivingcurrent, the semiconductor optical amplifier 2 can be used as a switchof high-speed light so as to correspond to even the pulse lightfrequency of the semiconductor laser 1.

When the recording apparatus 200 has a configuration in which laserlight with, for example, a 407 nm wavelength is emitted from thesemiconductor laser 1, the semiconductor optical amplifier 2 is alsomade of the same material as that of a blue-purple semiconductor laserthat emits light with the same wavelength as that of the active layer24, a guide layer, a clad layer, or the like.

The beam shaping unit 240 is disposed on the rear stage of the opticalamplifier unit 230.

The beam shaping unit 240 includes a lens 10 and a prism (for example,an anamorphic prism) 11 that shapes the beam of the laser light.

The laser light shaped by the beam shaping unit 240 is emitted to anoptical recording medium 21 via an optical pickup (not shown).

The variable voltage source 12 supplies the bias voltage Vsa to thesub-electrode 114 of the saturable absorber section 117 of thesemiconductor laser 1 of the mode-lock laser unit 210.

The bias Tee configured by the coil 13 and the capacitor 14 supplies again current Ig to the main electrode 113 of the gain section 116 of thesemiconductor laser 1 of the mode-lock laser unit 210. An AC componentand a DC component of the gain current Ig are supplied from thereference signal generation unit 15 to the capacitor 14 and the coil 13,respectively.

The reference signal generation unit 15 generates the master clocksignal. The master clock signal generated by the reference signalgeneration unit 15 is transmitted to the data generation unit (recordingsignal generation unit) 16.

The reference signal generation unit 15 supplies, to the capacitor 14 ofthe bias Tee, a signal synchronized with the master clock signal as theAC component of the gain current Ig for the semiconductor laser 1.

The data generation unit (recording signal generation unit) 16 generatesa data pulse by loading recording data in synchronization with themaster clock signal. The generated data pulse is transmitted to the SOAdrive circuit 17.

The SOA drive circuit 17 generates a driving current of thesemiconductor optical amplifier (SOA) 2 based on the data pulse. Thedriving current is supplied to the semiconductor optical amplifier 2 ofthe optical amplifier unit (SOA unit) 230.

The signal synchronized with the master clock signal is transmitted fromthe reference signal generation unit 15 to the spindle circuit 18.

The driving current is supplied from the spindle circuit 18 to thespindle motor 20, so that the rotation speed of the disc-shaped opticalrecording medium 21 is controlled by the driving current of the spindlemotor 20.

The signal synchronized with the master clock signal is supplied as theAC component of the gain current Ig for the semiconductor laser 1, theoptical pulse of the laser light emitted from the semiconductor laser 1can be synchronized with the master clock signal.

Further, since the driving current of the semiconductor opticalamplifier (SOA) 2 is generated based on the data pulse generated byloading the recording data in synchronization with the master clocksignal, the semiconductor optical amplifier 2 is driven insynchronization with the master clock signal. In this way, themodulation (ON or OFF) of the laser light by the semiconductor opticalamplifier 2 can be synchronized with the master clock signal.

Furthermore, since the signal synchronized with the master clock signalis transmitted to the spindle circuit 18 that supplies the drivingcurrent to the spindle motor 20, the rotation driving of the opticalrecording medium by the spindle motor 20 can be synchronized with themaster clock signal.

In the recording apparatus 200 according to the above-describedembodiment, the reference signal generation unit 15 supplies the signalsynchronized with the master clock signal to the gain section 116 of thesemiconductor laser 1. Thus, the optical pulse of the laser lightemitted from the semiconductor laser 1 can be synchronized with themaster clock signal.

The data generation unit 16 generates the data pulse by loading therecording data based on the master clock signal and the SOA drivecircuit 17 generates the current pulse of the driving current of thesemiconductor optical amplifier 2 based on the data pulse.

Since the semiconductor optical amplifier 2 is driven by the currentpulse of the driving current, the semiconductor optical amplifier 2 isdriven to be turned on or off in synchronization with the master clocksignal, so that the laser light is modulated in synchronization with themaster clock signal.

Due to the synchronization with the master clock signal, the opticalpulse of the laser light and the modulation of the laser light can besynchronized with each other.

Thus, since the optical pulse of the laser light and the modulation ofthe laser light can be synchronized with each other, the optical pulseof the laser light and the modulation of the laser light can easily besynchronized with each other even when the laser light has a very highpulse optical frequency.

Accordingly, in the recording apparatus, the recording can be accuratelyrealized with a high density and at a high speed.

Due to the synchronization with the master clock signal, the modulationof the laser light by the semiconductor optical amplifier 2 and therotation of the optical recording medium 21 can be synchronized witheach other.

Thus, since information can be recorded with an appropriate density at adesired position on the disc-shaped optical recording medium 21, therecording can be accurately realized with a high density and at a highspeed.

2. Second Embodiment

However, even when the oscillation pulse of the semiconductor laser 1and the data pulse are synchronized with each other, the amplitude ofthe output light may become unstable due to the change in the time axisin the rising point and the falling point of the pulse of the drivingcurrent of the semiconductor optical amplifier 2.

That is, the amount of current is considerably changed in transitionregions (a portion indicated by diagonal lines) near the rising pointand the falling point of the pulse of the driving current of thesemiconductor optical amplifier 2 shown in FIG. 5B, which is generatedbased on the data pulse shown in FIG. 5A. Therefore, the current amountis not sufficient.

For this reason, in output light obtained from input light shown in FIG.5C, as shown in FIG. 5D, its amplitude of the portions (portionsindicated by diagonal lines) corresponding to the transition regionsnear the rising point and the falling point of the pulse of the drivingcurrent of the semiconductor optical amplifier 2 may decrease.

An embodiment of a recording apparatus configured to resolve such aproblem will be described below.

The configuration of the recording apparatus according to a secondembodiment is schematically illustrated in FIG. 6.

A recording apparatus 300 shown in FIG. 6 includes a phase adjustmentcircuit 19 disposed between the data generation unit (the recordingsignal generation unit) 16 and the SOA drive circuit 17.

The phase adjustment circuit 19 adjusts the phase of the data pulsegenerated by the data generation unit (the recording signal generationunit) 16 and then supplies the data pulse to the SOA drive circuit 17.

Thus, the influence of the rising point and the falling point of thepulse of the driving current of the semiconductor optical amplifier 2 onthe output light is reduced.

The adjustment of the phase of the driving current by the phaseadjustment circuit 19 will be described with reference to FIGS. 7A to7D.

A data pulse shown in FIG. 7A is adjusted by the phase adjustmentcircuit 19 so as to be shifted backward by a period corresponding to apart of the pulse width, as shown in FIG. 7B. The data pulse ispreferably shifted by about the half of a period of laser light input tothe semiconductor optical amplifier 2 shown in FIG. 7D.

Thus, in the vicinity (transition regions) of the rising point and thefalling point of the pulse of the SOA driving current shown in FIG. 7C,the intensity of the light input to the semiconductor optical amplifier2 shown in FIG. 7D decreases.

Accordingly, as the waveform of the light output from the semiconductoroptical amplifier 2 is shown in FIG. 7E, the optical pulse withsufficient amplitude can be obtained from the output light without theinfluence of the rising point and the falling point of the driving pulseon the output light.

The remaining configuration of the recording apparatus 300 shown in FIG.6 is the same as that of the recording apparatus 200 shown in FIG. 1according to the first embodiment. The same reference numerals are givenand the description thereof will not be repeated.

In the recording apparatus 300 according to the above-described secondembodiment, the optical pulse of the laser light and the modulation ofthe laser light can easily be synchronized with each other, as in therecording apparatus 200 according to the first embodiment.

Thus, the optical pulse of the laser light and the modulation of thelaser light can be synchronized with each other, even when the laserlight has a very high pulse optical frequency.

Accordingly, in the recording apparatus, the recording can be accuratelyrealized with a high density and at a high speed.

As in the recording apparatus 200 according to the first embodiment, themodulation of the laser light by the semiconductor optical amplifier 2and the rotation of the optical recording medium 21 can be synchronizedwith each other.

Thus, since information can be recorded with an appropriate density at adesired position on the disc-shaped optical recording medium 21, therecording can be accurately realized with a high density and at a highspeed.

In the recording apparatus 300 according to the second embodiment, thephase adjustment circuit 19 is provided between the data generation unit16 and the SOA drive circuit 17. The phase adjustment circuit 19 adjuststhe phase of the data pulse generated by the data generation unit 16 andthen supplies the data pulse to the SOA drive circuit 17.

Further, the phase adjustment circuit 19 adjusts the phase of therecording signal so as to prevent the light emission pulse of the laserlight from overlapping with the rise and fall transition regions of thedriving pulse of the semiconductor optical amplifier 2. Thus, since theinfluence of the transition regions of the driving pulse on themodulated laser light can be eliminated, it is possible to obtain theoptical pulse with sufficient amplitude.

3. Third Embodiment

As a method of improving the transmission rate of the optical recordingmedium, there is a method of providing a plurality of optical pickupsand configuring a plurality of laser beams to be emitted to the opticalrecording medium in order to increase data channels.

For example, the recording region of the optical recording medium isdivided into several zones (zoning) and assigning the zones to theplurality of optical pickups.

A case where the zoning is performed on the optical recoding medium 21is shown in FIG. 8.

As shown in FIG. 8, the optical recording medium 21 is divided into theplurality of zones 21A, 21B, and 21C with a doughnut shape in a radiusrange from the center of the optical recording medium 21 and assigningthe zones 21A, 21B, and 21C to two optical pickups 31 and 32.

With such a configuration, when the rotation number of the opticalrecording medium 21 is set to be constant, it is necessary to set eachclock of the inner circumference and the outer circumference of eachzone of the optical recording medium 21, so that the recording densityis constant, in order to realize a linear density in all the zones.

For example, when the frequency of the master clock is 1713.60 MHz, theminimum clock density is 66 nm/clock, and the rotation number of theoptical recording medium is 3600 rpm, a setting example of each clock ofthe inner circumference and the outer circumference of each zone isshown in Table 1. In Table 1, R denotes a radius from the center, Zonedenotes a zone number, V1 denotes a linear velocity, Clk denotes a clockfrequency, Inner denotes the clock density of the inner circumference ofthe zone, and Outer denotes the clock density of the outer circumferenceof the zone. Further, Ratio denotes a ratio at which the frequency ofthe master clock is eliminated by the clock frequency of each zone.

TABLE 1 R (mm) Zone VI (m/s) Clk (MHz) Inner Outer Ratio 30 0 11.31171.36 66.00 70.40 10.00 32 1 12.06 182.78 66.00 70.13 9.38 34 2 12.82194.21 66.00 69.88 8.82 36 3 13.57 205.63 66.00 69.67 8.33 38 4 14.33217.06 66.00 69.47 7.89 40 5 15.08 228.48 66.00 69.30 7.50 42 6 15.83239.90 66.00 69.14 7.14 44 7 16.59 251.33 66.00 69.00 6.82 46 8 17.34262.75 66.00 68.87 6.52 48 9 18.10 274.18 66.00 68.75 6.25 50 10 18.85285.60 66.00 68.64 6.00 52 11 19.60 297.02 66.00 68.54 5.77 54 12 20.36308.45 66.00 68.44 5.56 56 13 21.11 319.87 66.00 68.36 5.36 58 14 21.87331.30 66.00 68.28 5.17 60 15 22.62 342.72 66.00 68.20 5.00

When the clock Clk of each zone is set, as in Table 1, the clock densityof the inner circumference of each zone is constantly set to 66(nm/clock) and the recording density of the inner circumference wherethe recording density is the highest in the zone can be made to beconstant.

In a case of the MOPA optical source using the mode-lock laser, theclock frequency is determined by the length of a resonator of themode-lock laser. Accordingly, as in the example shown in Table 1, it isdifficult to minutely optimize the clock frequency of each zone.

In order to resolve this problem, the clock frequency of each zone isset at an integer ratio so as to be easily generated from the samemaster clock by signal processing or the like.

Further, the clock frequency of each zone is set at an integer ratiowithin a range in which the recording density of the zone does notexceed the limit so that the clock frequency is slightly high.

A setting example in which the clock frequency is set in this manner isshown in Table 2.

TABLE 2 Integer R (mm) Zone VI (m/s) Ratio Clk (MHz) Inner Outer 30 011.31 10 171.36 66.00 70.40 32 1 12.06 10 171.36 70.40 74.80 34 2 12.829 190.40 67.32 71.28 36 3 13.57 9 190.40 71.28 75.24 38 4 14.33 8 214.2066.88 70.40 40 5 15.08 8 214.20 70.40 73.92 42 6 15.83 8 214.20 73.9277.44 44 7 16.59 7 244.80 67.76 70.84 46 8 17.34 7 244.80 70.84 73.92 489 18.10 7 244.80 73.92 77.00 50 10 18.85 6 285.60 66.00 68.64 52 1119.60 6 285.60 68.64 71.28 54 12 20.36 6 285.60 71.28 73.92 56 13 21.116 285.60 73.92 76.56 58 14 21.87 6 285.60 76.56 79.20 60 15 22.62 5342.72 66.00 68.20

In the example shown in Table 2, the zones of the optical recordingmedium 21 are set by integers and the clock frequency of each zone isset at an integer ratio. That is, on the assumption that the integerratios are n (integers from 5 to 10), a relationship of “the clockfrequency=1713.6/n” is satisfied.

Thus, when the clock frequency is set at the integer ratio, the clockfrequency can easily be generated from the same master clock by signalprocessing or the like.

The clocks and the clock densities are compared in the innercircumference and the outer circumference of the zone in the examples ofTable 1 and Table 2 and the result is shown in FIG. 9. In FIG. 9 adashed line indicates the case of Table 1 and polygonal lines indicatedby signs •, ◯ and Δ indicate the case of the Table 2.

An embodiment of a recording apparatus with such a configuration will bedescribed below.

The configuration of a recording apparatus according to a thirdembodiment is shown in FIG. 10.

A recording apparatus 400 shown in FIG. 10 includes a zonefrequency-divider circuit 35 disposed between the reference signalgeneration unit 15 and the data generation unit (the recording signalgeneration unit) 16.

The zone frequency-divider circuit 35 generates a clock signal of eachzone of the optical recording medium 21 having the relationship of aninteger ratio between the frequency of the master clock signal and thefrequency of the clock signal using the master clock signal generated bythe reference signal generation unit 15. Then, the clock signal of eachzone generated by the zone frequency-divider circuit 35 is supplied tothe data generation unit (the recording signal generation unit) 16.

The clock signal of each zone generated in this way is reflected on thedata pulse or the driving current of the semiconductor optical amplifier(SOA) 2. When the semiconductor optical amplifier 2 is turned on or off,the laser light from the semiconductor laser 1 is supplied to theoptical pickup 31 in synchronization with the clock signal of each zone.Thus, information can be recorded at an appropriate clock in each zoneof the optical recording medium 21.

In FIG. 10, only one optical pickup 31 is illustrated. However, as shownin FIG. 8, the plurality of optical pickups 31 and 32 are assigned tothe zones.

The remaining configuration of the recording apparatus 400 shown in FIG.10 is the same as that of the recording apparatus 300 shown in FIG. 6according to the second embodiment. The same reference numerals aregiven and the description thereof will not be repeated.

In the recording apparatus 400 according to the above-described thirdembodiment, the optical pulse of the laser light and the modulation ofthe laser light can easily be synchronized with each other, as in therecording apparatuses 200 and 300 according to the first and secondembodiments.

Thus, the optical pulse of the laser light and the modulation of thelaser light can be synchronized with each other, even when the laserlight has a very high pulse optical frequency.

Accordingly, in the recording apparatus, the recording can be accuratelyrealized with a high density and at a high speed.

As in the recording apparatuses 200 and 300 according to the first andsecond embodiments, the modulation of the laser light by thesemiconductor optical amplifier 2 and the rotation of the opticalrecording medium 21 can be synchronized with each other.

Thus, since information can be recorded with an appropriate density at adesired position on the disc-shaped optical recording medium 21, therecording can be accurately realized with a high density and at a highspeed.

As in the recording apparatus 300 according to the second embodiment,the phase adjustment circuit 19 adjusts the phase of the recordingsignal so as to prevent the light emission pulse of the laser light fromoverlapping with the rise and fall transition regions of the drivingpulse of the semiconductor optical amplifier 2.

Thus, since the influence of the transition regions of the driving pulseon the modulated laser light can be eliminated, it is possible to obtainthe optical pulse with a sufficient amplitude.

Further, in the recording apparatus 400 according to the thirdembodiment, the zone frequency-divider circuit 35 is disposed betweenthe reference signal generation unit 15 and the data generation unit 16.The zone frequency-divider circuit 35 generates the clock signal of eachzone of the optical recording medium 21 having the relationship of theinteger ratio between the frequency of the master clock signal and thefrequency of the clock signal.

Accordingly, the clock signal of each zone can easily be generated.Therefore, even when the mode-lock oscillation type laser is used as anoptical source, the same linear density can be set in the respectivezones under the constant rotation number of the optical recording medium21 by setting the recording density of each zone to be as high aspossible.

In the above-described third embodiment, the zone frequency-dividercircuit 35 is further provided in the configuration of the secondembodiment in which the phase adjustment circuit 19 is included. Thezone frequency-divider circuit may be provided in the configuration ofthe first embodiment in which no phase adjustment circuit is included.

In the recording apparatus according to each embodiment describedembodiment, the optical recording medium 21 to be used has a click shapeand information is recorded by rotating the optical recording medium 21by the spindle motor 20. However, the embodiments are applicable to arecording apparatus with other configurations.

For example, information may be recorded in an optical recording mediumwith a card shape by scanning laser light to the optical recordingmedium.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope and without diminishing itsintended advantages. It is therefore intended that such changes andmodifications be covered by the appended claims.

The application is claimed as follows:
 1. A recording apparatus, whichrecords information in an optical recording medium, comprising: amode-lock laser unit including a saturable absorber section that appliesa bias voltage, a gain section that feeds a gain current, asemiconductor laser that emits laser light used to record theinformation on the optical recording medium, and an external resonator;an optical modulation unit performing amplification modulation on thelaser light emitted from the mode-lock laser unit; a reference signalgeneration unit generating a master clock signal and supplying a signalsynchronized with the master clock signal to the gain section of thesemiconductor laser; a recording signal generation unit generating arecoding signal based on the master clock signal; and a driving circuitgenerating a driving pulse used to drive the optical modulation unitbased on the recording signal.
 2. The recording apparatus according toclaim 1, further comprising: a phase adjustment circuit that is disposedbetween the reference signal generation unit and the driving circuit andadjusts a phase of the recording signal so as to prevent a lightemission pulse of the laser light from overlapping with rise and falltransition regions of the driving pulse.
 3. The recording apparatusaccording to claim 1, wherein the optical recoding medium has a discshape and rotation of the optical recording medium is controlled by acontrol signal synchronized with the master clock signal.
 4. Therecording apparatus according to claim 3, further comprising: a zonefrequency-divider circuit generating a clock signal with a frequency setat an integer ratio with respect to a frequency of the master clocksignal in each of a plurality of zones formed by dividing a recodingregion, where information is recorded in the optical recording medium,in a radius range from a center of the optical recording medium.